1. Field of the Invention
The present invention relates to method for threading a wire in a wire discharge machine.
2. Description of the Prior Art
In a wire discharge machine, when the processing of one section of a workpiece is completed and the process is moved to another section for processing, it is necessary to cut the wire electrode and perform a so-called wire threading operation wherein the wire electrode is inserted into a starting hole for a new processing section. In the threading operation, a wire electrode is cut by passing a current through it to melt the wire. However, a winding pattern is generally imprinted in the wire electrode when it is wound on a bobbin, and this pattern remains in the melted wire electrode. Accordingly, feeding the wire through a small-diameter starting hole into a lower wire guide is not necessarily easy.
For this reason, various methods such as those shown in FIG. 1a, 1b, and 1c are used conventionally. In FIG. 1a, a straight-through pipe guide P for guiding the wire only at the time when it is to be threaded is positioned directly under an upper wire guide g and the wire electrode 1 is fed through by a wire feed roller r to penetrate a workpiece W. In FIG. 1b, a small-diameter orifice o is provided below the upper wire guide g. Process liquid is sprayed through the orifice o, and while the wire electrode 1 is still covered with the process liquid it is fed by the wire feed roller r to penetrate the workpiece W. In FIG. 1c, the upper wire guide g provided on the lower end of a pipe guide g is moved close to the upper surface of the workpiece W and the wire electrode 1 is fed through by the wire feed roller r to penetrate the workpiece W.
In such conventional methods the following types of problems arise. Specifically, in the method of FIG. 1a, the pipe guide is divided into two parts in the vertical direction so as for each of the parts to advance toward and retreat from the processing section before and after the thread operation. Thus, the structure is complicated. In particular, considerable time and trouble is expended in maintenance. In the method shown in FIG. 1b, when the tapering process is performed, the lower end of the orifice obstructs the wire electrode. Thus the angle of taper is limited. In the method of FIG. 1c, the rigidity of the pipe guide g which maintains the lower wire guide g is low so that accurate positioning of the wire guide is difficult.